Various physiological (e.g., relating to heart function and/or respiration) and behavioral signals are frequently utilized to provide insight into the nervousness or truthfulness of the subject being monitored. For example, in polygraphy, a subject's fidgeting and/or muscle tensing activity is measured by means of an “activity pad” located on the seat the subject is sitting upon while being questioned by the polygrapher, thus monitoring any attempts by the subject to defeat the host of other sensors to which he/she is physically connected.
One useful physiological signal is the ballistocardiogram (BCG), which is a measure of the mechanical reaction of the human body to the pumping of blood therein. Specifically, the BCG measures mechanical activity of the heart and surrounding tissue, e.g., the impact of blood colliding with portions of the heart and blood vessels, recording the transient forces as minute changes in the weight of the subject. Unfortunately, the utility of BCG has remained quite limited in practice, as accurate measurements typically require the subject to be lying down and strapped to a complicated bed-like apparatus in order to isolate the BCG signal from gravitational effects. BCG measurement systems have even required the subject to hold his/her breath during data acquisition.
For a variety of tasks, including security screening, surveillance, and high-throughput medical screening, the ability to quickly and efficiently measure physiological signals such as the BCG, heart rate, respiration rate, and weight, as well as behavioral signals such as center-of-gravity (“CG”) changes, for human subjects would prove quite useful. For example, changes in behavioral signals may indicate nervous jitter (or fidgeting) behavior. Similarly, a sudden decrease in such movements may indicate a behavioral response termed “hypervigilance” or the “freeze response.” Furthermore, the ability to make such measurements unobtrusively would facilitate screening throughput (i.e., requiring no delays for subjects to undress or have electrodes placed on their skin) and reduce anxiety, embarrassment, or distraction due to awareness of the measurement system, any of which could influence the quality of the measured data.